Current sensing device for a multi-phase switched voltage regulator

ABSTRACT

A current sensor includes input modules each including an amplifier having a first input coupled to a respective winding of a regulator; a transistor having a control input coupled to the amplifier output, a first terminal coupled to a second input of the operational amplifier, and a second terminal configured to provide a sense current representing an unbalance of a phase current with respect to an average current of a winding; a current generator configured to bias the transistor; and an output resistor coupling the first terminal to an intermediate node. An output stage includes an output amplifier having a first input coupled to the intermediate node, and an output transistor having a control terminal coupled to the output amplifier output, a first terminal coupled to a second input of the output amplifier, and a second terminal providing another sense current representing a total current of the regulator.

BACKGROUND

1. Technical Field

The present disclosure relates to current sensing, and more particularlyto a current sensing device for a multi-phase switched voltage regulatorcapable of generating sense currents representing the current unbalanceof the phases in respect to the average current delivered by each phase,and the total current delivered by the voltage regulator.

2. Description of the Related Art

Multiphase voltage regulators are used in numerous applications, forexample as power supplies in microprocessors for personal computers,workstations, servers, printers and other similar electronic equipment.

Voltage regulators allow a desired output voltage to be providedproportionally to the current required by the supplied load. Thisfeature is commonly called “droop function” or “voltage positioning”.

The droop function allows current sense devices to read or estimate thecurrent delivered by the regulator. In general, current sense devicesread the current as a voltage drop across a sense resistance, that maybe a parasitic element of the voltage regulator. For example, in powerswitches, the sense resistance may be the on resistance of the switch orthe parasitic resistance of the phase inductor (phase winding).Alternatively, the resistance may be a series of components purposelyinserted in the circuit.

If a discrete sense resistance is used, a very precise current readingmay be provided. Moreover, using constantan-made resistances, thecurrent reading is almost independent from temperature variations.

Nevertheless, a drawback of this approach consists in being expensiveand in lowering current conversion efficiency.

Using parasitic resistances of the switches of the regulator for sensingthe delivered current is economical because no separate element is addedto the regulator. Unfortunately, the current sensing may be less precisebecause of the spread of the value of the conduction resistance of eachintegrated switch and to the variations produced by fluctuations of theworking temperature of the regulator.

Sensing the phase currents by exploiting the parasitic resistance of thephase winding is preferred because the reading tolerance may becontained within about 5%.

In general, a multiphase voltage regulator with N switches outputs asaw-tooth current with a period corresponding to T/N, wherein T is theswitching period of the phases. Nevertheless, switched voltageregulators use a control circuit for controlling the phase differencesbetween the input currents of the N switches, in order to balance thecurrent delivered by each phase and attain an effective current sharingmode of operation.

In currently available voltage regulators, the presence of particularlyprecise components to meet stringent specifications, poses furtherproblems. In particular, the ability of using ever smaller inductorsmakes the values of parasitic series resistances R_(L) to be exploitedas sense resistances comparable to the values of parasitic resistanceson PC application boards, because of the unavoidable resistance of themetal vias therein.

As emphasized in FIG. 1, for a dual-phase regulator, the presence of theparasitic resistances Rp1 in series with the resistances R_(L) impliesthat a current sensing circuit capable of discriminating the currentsense information on the phase winding is used for correct sensing. Thesolution is to implement a so-called fully differential reading.

In a known device described in U.S. Pat. No. 5,982,160 an R-C currentsense series connection is connected in parallel to each output inductorof the voltage regulator, as shown in FIG. 2. The values of the shuntresistor R and of the filter capacitor C of the R-C series connectionare determined such to match the time constant of the circuit R_(L)-L(the phase winding and its parasitic series resistance) with the timeconstant of the R-C series connection. In this matching condition, itmay be assumed that the DC component of the voltage drop on the phasewinding be equal to the voltage on the filter capacitor of the R-Cseries connection.

Moreover, the current sense circuit current signal of each R-C networkis analyzed together with the output signal through a resistance R_(G)by a controller. In the case represented in FIG. 2 b, a circuit is shownfor estimating the output current of the dual-phase voltage regulatoremphasizing that the controller has two dedicated pins for each R-Cseries connection.

The voltage regulator according to the prior approach, althoughadvantageous under several points of view, requires a controller with2*N pins for the total current reading, wherein N is the number ofphases of the voltage regulator. This increases complexity of theregulator and silicon area requirement.

A known two-phase voltage regulator is described in U.S. Pat. No.6,683,441 B2 and is shown in FIG. 3. The voltages of the two nodes,PHASE1 and PHASE2, are added by two resistances Rp connected in commonand sent to an input of an operational amplifier, and the other inputreceives the output voltage of the regulator. In this case, a fullydifferential current reading cannot be provided because the outputvoltage VOUT is used as a reference voltage by the operationalamplifier. Therefore, different parasitic resistances between the outputof the regulator and each phase cause an error in sensing the deliveredoutput current. Moreover, this prior architecture does not allow tosense each phase current.

Another known current sense circuit of a multi-phase voltage regulatoris disclosed in the published U.S. patent application No. 2008/0169797in the name of the same applicant, and is shown in FIG. 4. The currentsense circuit composed by the operational amplifier and the controlledMOSFET uses only two pins for reading the mean current flowing throughthe inductances of the N phases of the voltage regulator.

Unfortunately, also this prior architecture does not allow to sense eachphase current, neither a refined control of the current delivered by thevoltage regulator by correcting the current unbalancing in each phase.

BRIEF SUMMARY

A novel current sensing device for sensing the total current and thecurrent unbalance of each single phase in respect to the average currentdelivered by the same phase of a multiphase switched voltage regulatoris proposed herein.

The current sensing device according to one embodiment of the presentdisclosure has a modular architecture and is composed of as many inputmodules as the phases of the voltage regulator and is adapted togenerate as many sense currents representative of the difference betweenany single phase current and the average current delivered by the samephase.

According to one embodiment of this disclosure, each phase winding isconnected in parallel to a respective R-C series connection of a shuntresistor with a filter capacitor, and the common node between the shuntresistors and the filter capacitors are accessible. Each input modulecomprises:

-   -   an operational amplifier having an input coupled to the common        node of the R-C series connection of the respective phase,    -   a transistor controlled by the operational amplifier, having a        first current terminal shorted to the other input of the        operational amplifier, the sense current flowing through the        transistor and being delivered through the second current        terminal thereof,    -   a bias constant current generator connected to the first current        terminal of the transistor,    -   an output resistor coupling the first terminal to an        intermediate node of the current sensing device shared in common        by all the modules.

The sense current Idroop representative of the total current deliveredby the regulator is generated in one embodiment by an output stageincluding:

-   -   an output operational amplifier having a first input shorted to        said intermediate node,    -   an output transistor controlled by said output operational        amplifier, having a first current terminal shorted to a second        input of the output operational amplifier, said sense current        flowing through said output transistor and being delivered        through a second current terminal thereof,    -   a number N, equal to the number of phases of the voltage        regulator, of identical sense resistors connected in common to        said first current terminal of the output transistor and each        connected to the winding of a respective phase of the regulator.

Preferred embodiments of the sensing device are defined in the dependentclaims.

An innovative multi-phase switched voltage regulator, comprises phaseinductors each connected in parallel to a respective R-C seriesconnection of a shunt resistor with a filter capacitor, the common nodebetween the shunt resistor and the filter capacitor of each R-C seriesconnection being accessible, and the novel current sensing device.

The claims as filed are integral part of this description and are hereinincorporated by reference.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 basically depicts the equivalent circuit of a classic voltageregulator.

FIG. 2 a depicts a phase of a prior voltage regulator disclosed in thedocument U.S. Pat. No. 5,982,160.

FIG. 2 b depicts a phase of a prior voltage regulator and its currentsensing device disclosed in the document U.S. Pat. No. 5,982,160.

FIG. 3 depicts a prior two-phases voltage regulator and its currentsensing device disclosed in the document U.S. pat. No. 6,683,441.

FIG. 4 depicts a prior current sensing device of a two-phases voltageregulator disclosed in the document U.S. 2008/169797.

FIG. 5 shows a phase of a voltage regulator and an innovative currentsensing device for sensing in a differential way each phase current andthe output current delivered by the regulator.

DETAILED DESCRIPTION

In the ensuing description reference is made to a two-phase voltageregulator 10, shown in FIG. 5, though the same considerations applymutatis mutandis for a generic N-phase voltage regulator. The voltageregulator 10 includes first and second phases 12, 14 that are controlledby a control circuit 16 and produce first and second regulated outputvoltages V_(OUT1), V_(OUT2) (generically V_(OUTi)) on first and secondoutput terminals 18, 20, respectively. The first phase 12 includes afirst bridge driver circuit 21, a first winding 22, with a resistor 24of value R_(L) representing the parasitic series resistance of thewinding 22, coupled in parallel with a series-connection that includes ashunt resistor 26 of resistance value R_(CS) and a filter capacitor 28of capacitance value C_(CS) coupled to each other at a first common nodeCS1P. The second phase 14 includes a second bridge driver circuit 29, asecond winding 30, with a resistor 32 of value R_(L) representing theparasitic series resistance of the winding 30, coupled in parallel witha series-connection that includes a shunt resistor 34 of resistancevalue R_(CS) and a capacitor 36 of capacitance value C_(CS) coupled toeach other at a second common node CS2P.

A novel architecture of a current sensing device 38 for the switchingvoltage regulator 10 is shown in FIG. 5 together with the two phases 12,14 of the regulator and the control circuit 16, in order to show how thecurrent sensing device 38 is connected.

In general, the novel current sensing device 38 comprises a number N ofinput modules 40, 42, with N being equal to the number of phases of thevoltage regulator 10, each comprising an operational amplifier OA1, OA2having a first input (that, in the embodiment of FIG. 5, is thenon-inverting input) connected to the common node CS1P, CS2P(generically CSiP) of the corresponding R-C series connection betweenthe shunt resistance 26, 34 and the filter capacitance 28, 36 of therespective phase 12, 14 of the regulator 10. Each input module 34, 36also includes a transistor M1, M2 (generically Mi), that in theexemplary embodiment of FIG. 5 is a MOSFET, biased with a fixed biascurrent Ibias by a current generator 43, 44 connected to a first currentterminal of the respective transistor M1, M2.

Each transistor M1, M2 controlled by the respective operationalamplifier OA1, OA2 is connected to deliver, through a second currentterminal (that in the exemplary embodiment of FIG. 5 is the drain), acurrent I_(INFOCS1), I_(INFOCS2) (generically I_(INFOCSi))representative of the current unbalance of the respective phase currentI_(PHASE1), I_(PHASE2) (generically I_(PHASEi)) with respect to theaverage current delivered by each phase.

The input operational amplifiers OA1, OA2 replicate the voltage appliedon their first input on their second input (that in the exemplaryembodiment is the inverting input) connected to the first currentterminal (for example, the source) of the respective transistor M1, M2they control, that is:VCSiP=I _(PHASEi) *R _(L) +V _(OUTi).

The first current terminals of all transistors Mi are coupled togetherto a common intermediate node CSPM at a voltage VCSPM through identicalresistances R_(S).

In the embodiment of FIG. 5 in which only two phases are considered, thevoltage VCSPM is:

${VCSPM} = {{\frac{I_{{PHASE}\; 1} + I_{{PHASE}\; 2}}{2} \cdot R_{L}} + {\frac{V_{{OUT}\; 1} + V_{{OUT}\; 2}}{2}.}}$Therefore, the currents I_(INFOCS1) and I_(INFOCS2) delivered by thetransistors M1 and M2 are

$I_{{INFOCS}\; 1} = {{Ibias} + {\frac{I_{{PHASE}\; 1} - I_{{PHASE}\; 2}}{2\; R_{S}} \cdot R_{L}} + \frac{V_{{OUT}\; 1} - V_{{OUT}\; 2}}{2\; R_{S}}}$$I_{{INFOCS}\; 2} = {{Ibias} - {\frac{I_{{PHASE}\; 1} - I_{{PHASE}\; 2}}{2\; R_{S}} \cdot R_{L}} - {\frac{V_{{OUT}\; 1} - V_{{OUT}\; 2}}{2\; R_{S}}.}}$

With I_(M) being the average current delivered by each phase of theregulator,

$I_{M} = \frac{I_{{PHASE}\; 1} + I_{{PHASE}\; 2}}{2}$ then$I_{{INFOCS}\; 1} = {{Ibias} + {\frac{R_{L}}{R_{S}} \cdot I_{{PHASE}\; 1}} - {\frac{R_{L}}{R_{S}} \cdot I_{M}} + \frac{V_{{OUT}\; 1} - V_{{OUT}\; 2}}{2\; R_{S}}}$$I_{{INFOCS}\; 2} = {{Ibias} + {\frac{R_{L}}{R_{S}} \cdot I_{{PHASE}\; 2}} - {\frac{R_{L}}{R_{S}} \cdot I_{M}} - {\frac{V_{{OUT}\; 1} - V_{{OUT}\; 2}}{2\; R_{S}}.}}$Therefore, I_(INFOCS1) and I_(INFOCS2) may be considered proportional tothe current unbalance of the respective phase current with respect tothe average current delivered by each phase, provided that the errorterm

$\frac{V_{{OUT}\; 1} - V_{{OUT}\; 2}}{2\; R_{S}}$is negligible.

The error term is due to eventual mismatches between the parasiticresistances Rp1 and Rp2 of the lines that connect the output nodes 18,20 of the phases, at the voltages V_(OUT1) and V_(OUT2), to the commonoutput terminal of the voltage regulator. Usually the differenceV_(OUT1) -V_(OUT2) is relatively small because these parasiticresistances may be accurately matched to each other, thus the error termis in practice negligible.

The current sensing device 12 has an output stage 45 that senses in adifferential manner the total current delivered by the voltage regulator10. Such an output stage 45 comprises an operational amplifier OOA, atransistor MO, and identical resistances R_(G) connected between acommon terminal CSN and the output nodes 18, 20 of the respective phasesof the regulator 10. The operational amplifier OOA controls thetransistor MO, which delivers to the control circuit 16 a currentI_(DROOP) representative of the total current delivered by the voltagegenerator The amplifier OOA replicates on the terminal CSN the voltageVCSPM, thus the current I_(DROOP) is

$\begin{matrix}{I_{DROOP} = {{\frac{I_{{PHASE}\; 1} + I_{{PHASE}\; 2}}{R_{G}} \cdot R_{L}} + \frac{V_{{OUT}\; 2} - V_{{OUT}\; 1}}{R_{G}} +}} \\{\frac{V_{{OUT}\; 1} - V_{{OUT}\; 2}}{R_{G}}} \\{= {\frac{R_{L}}{R_{G}} \cdot ( {I_{{PHASE}\; 1} + I_{{PHASE}\; 2}} )}}\end{matrix}$i.e., it is proportional to the total current delivered by the voltageregulator.

The novel current sensing device 38 has N+1 pins (CSiP and CSN) and iscapable of generating in a fully differential manner a current I_(DROOP)representative of the total current delivered by the switching voltageregulator 10, and with good precision currents I_(INFOCS1), I_(INFOCS2)representative of the unbalance of the N phase currents. Therefore, itprovides the desired information to the control circuit 16 for drivingthe phases of the voltage regulator with an accurate current sharingcondition.

Even if the above description refers to a two-phase voltage regulator,the novel architectures may be easily adapted whichever the number N ofphases of the regulator is, simply by realizing as many identical inputmodules of the current sensing device as the number N of phases and bycoupling all nodes VCSiP to a same common node CSPM through identicalresistances R_(S). Similarly, the output stage of the current sensingdevice will comprise N identical resistances R_(G) connected in commonto the terminal CSN and to the respective phase winding as shown in FIG.5.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

The invention claimed is:
 1. A current sensing device for sensing phasecurrents of a multi-phase switched voltage regulator having phasewindings and R-C series couplings respectively coupled to the phasewindings, each R-C series coupling including a shunt resistor and afilter capacitor coupled to each other at a first intermediate node ofthe R-C series coupling, the current sensing device comprising: aplurality of input modules configured to be coupled to respective phasewindings of the regulator and configured to deliver sense currentsrepresenting unbalances of respective phase currents of the regulatorwith respect to an average current delivered by each phase winding, eachinput module including: an operational amplifier having a first inputconfigured to be coupled to said first intermediate node of the R-Cseries coupling of the respective phase, a transistor having a controlinput coupled to an output of said operational amplifier, a firstcurrent terminal coupled to a second input of the operational amplifier,and a second current terminal configured to provide a respective one ofsaid sense currents, a constant current generator to provide a constantcurrent and to bias said transistor through said first current terminal,and an output resistor coupling said first terminal to a secondintermediate node shared in common by all said modules; and an outputstage configured to deliver another sense current representative of atotal current delivered by the regulator, the output stage including: anoutput operational amplifier having a first input coupled to said secondintermediate node, an output transistor having a control terminalcoupled to an output of said output operational amplifier, a firstcurrent terminal coupled to a second input of the output operationalamplifier, and a second current terminal configured to provide saidanother sense current, and a plurality of sense resistors, equal innumber to the input stages, coupled in common to said first currentterminal of the output transistor and each configured to be coupled tothe respective phase winding of the regulator.
 2. The current sensingdevice of claim 1, wherein said first input of each operationalamplifier is a non-inverting input, and said second input of eachoperational amplifier is an inverting input.
 3. A multi-phase switchedvoltage regulator, comprising: a plurality of phase windings; aplurality of R-C series connections respectively coupled to the phasewindings; each R-C series connection including a shunt resistor and afilter capacitor coupled to each other at a first intermediate node; anda current sensing device that includes: a plurality of input modulescoupled respectively to the phase windings and configured to deliversense currents representing unbalances of respective phase currents ofthe regulator with respect to an average current delivered by each phasewinding, each input module including: an operational amplifier having afirst input coupled to said first intermediate node of the R-C seriescoupling of the respective phase, a transistor having a control inputcoupled to an output of said operational amplifier, a first currentterminal coupled to a second input of the operational amplifier, and asecond current terminal configured to provide a respective one of saidsense currents, a constant current generator to provide a constantcurrent and to bias said transistor through said first current terminal,and an output resistor coupling said first current terminal to secondintermediate node shared in common by all said modules; and an outputstage configured to deliver another sense current representative of atotal current delivered by the regulator, the output stage including: anoutput operational amplifier having a first input coupled to said secondintermediate node, an output transistor having a control terminalcoupled to an output of said output operational amplifier, a firstcurrent terminal coupled to a second input of the output operationalamplifier, and a second current terminal configured to provide saidanother sense current, and a plurality of sense resistors, equal innumber to the input stages, coupled in common to said first currentterminal of the output transistor and respectively coupled to the phasewindings.
 4. The multi-phase switched voltage regulator of claim 3,wherein each R-C series connection has a time constant that matches atime constant of the respective phase winding.
 5. The multi-phaseswitched voltage regulator of claim 3, wherein said first input of eachoperational amplifier is a non-inverting input, and said second input ofeach operational amplifier is an inverting input.
 6. A current sensingdevice for sensing phase currents of a multi-phase switched voltageregulator having phase windings, the current sensing device comprising:a plurality of input modules configured to be coupled to respectivephase windings of the regulator and configured to deliver sense currentsrepresenting unbalances of respective phase currents of the regulatorwith respect to an average current delivered by each phase winding, eachinput module including: an operational amplifier having a first inputconfigured to be coupled to said respective phase winding; a transistorhaving a control input coupled to an output of said operationalamplifier, a first current terminal coupled to a second input of theoperational amplifier, and a second current terminal configured toprovide a respective one of said sense currents; a constant currentgenerator to provide a constant current and to bias said transistorthrough said first current terminal; and an output resistor couplingsaid first terminal to an intermediate node shared in common by all saidmodules; and an output stage configured to deliver another sense currentrepresentative of a total current delivered by the regulator, the outputstage including: an output operational amplifier having a first inputcoupled to said intermediate node, and an output transistor having acontrol terminal coupled to an output of said output operationalamplifier, a first current terminal coupled to a second input of theoutput operational amplifier, and a second current terminal configuredto provide said another sense current.
 7. The current sensing device ofclaim 6, wherein said first input of each operational amplifier is anon-inverting input, and said second input of each operational amplifieris an inverting input.
 8. The current sensing device of claim 6, whereinthe output stage includes a plurality of sense resistors, equal innumber to the input stages, coupled in common to said first currentterminal of the output transistor and each configured to be coupled tothe respective phase winding of the regulator.